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NEW YORK — Shares of Pennsylvania-based ViroPharma jumped more than 25% in early trading Monday after the drugmaker that specializes in rare diseases was acquired by European-based Shire for about $4.2 billion.

Shire, which is listed on the London Stock Exchange and which trades on the Nasdaq, paid $50 per share for ViroPharma, a 27% premium over Friday's close of $39.78. ViroPharma shares were trading Monday at about $49.40, closer to the deal price. Shire shares were up 2.9% to $138.34 in early trading.

In a recurring trend in the competitive and lucrative world of pharmaceuticals where profitability is hurt when drug patents expire, Shire targeted ViroPharma because of its growth profile and its pipeline of potentially profitable drugs, including Cinryze, a drug that treats immune disorder hereditary angioedema.

Shire said the deal will enhance both its short- and long-term revenue growth profile and will result in cost savings of roughly $150 million by 2015.

ViroPharma's focus on treating rare diseases that effect a small number of patients was the main attraction, Shire executives said.

"The acquisition of ViroPharma will immediately benefit Shire and is entirely consistent with our clear strategic objective of strengthening our rare disease portfolio," Shire CEO Flemming Ornskov said in a statement. "It brings us a new growth driving product which augments our already strong growth prospects."

Vincent J. Milano, ViroPharma's CEO said: "We are pleased to announce our merger with Shire, which like ViroPharma, is focused on developing products for patients suffering from rare diseases. By joining with Shire, ViroPharma will become part of a larger, more diverse biopharmaceutical company and will benefit from Shire's innovation, scale and global reach."